DictionaryCoding: Foundation-only types limit non-Darwin compilation

[leogdion]

Summary

The DictionaryCoding module (in Packages/AtLeastKit/Sources/DictionaryCoding/) relies heavily on Foundation types that are not available on non-Darwin platforms:

• NSMutableDictionary, NSMutableArray, NSNull, NSNumber
• NSDecimalNumber, NSURL, NSDate, NSData
• CFGetTypeID, CFBooleanGetTypeID, CFUUID (Darwin-only CoreFoundation)

Impact

AtLeastKit targets cross-platform testing (macOS, iOS, watchOS, tvOS, visionOS). While all current targets are Apple platforms, the Foundation dependency means:

1. DictionaryCoding cannot be tested on Linux CI runners
2. If AtLeastKit ever needs to support non-Apple platforms, this module would need significant rework

Options to evaluate

1. Document the limitation — add a note that DictionaryCoding is Darwin-only by design
2. Add #if canImport(ObjectiveC) guards — provide a stub or alternative implementation for non-Darwin
3. Replace with an external package — evaluate existing Swift packages that solve Dictionary ↔ Codable bridging cross-platform

Context

Introduced in PR #172 (v1.0.0-beta.5) for WatchConnectivity message serialization via Messagable+Codable.